msm:ipa:mhi: fix the number of max channel for mhi

Reuse the same set of evt channel for auto/qmap flow control.
Fix the number of max channel and make sure number of event
channels are within the mhi_evid_limits.

Change-Id: I810da70251c1d9eb9f78a47a6994d1d44eb5eeb0
Signed-off-by: Bojun Pan <bojunp@codeaurora.org>